Liquid-crystal displays with active-matrix addressing in which the liquid-crystal layer has a homeotropic or tilted homeotropic alignment in the switched-off state, such as, for example, displays based on the ECB (electrically controlled birefringence) effect or the DAP (deformation of aligned phases) effect, are known. They were described for the first time in M. F. Schiekkel and K. Fahrenschon, Appl. Phys. Lett. 19, 3912 (1971).
However, liquid-crystal displays of this type have some disadvantages compared with known active-matrix TN displays, in particular a high viewing-angle dependence of the contrast and of the grey shades.
A newer variant of ECB displays is active-matrix displays based on the VAN (vertically aligned nematic) effect or the VAC (vertically aligned cholesteric) effect. VAN displays have been described, inter alia, in S. Yamauchi et al., SID Digest of Technical Papers, pp. 378ff (1989), and VAC displays have been described in K. A. Crabdall et al., Appl. Phys. Lett. 65, 4 (1994).
Like the ECB displays which were known earlier, the more-recent VAN and VAC displays contain a layer of liquid-crystalline medium between two transparent electrodes, where the liquid-crystalline medium has a negative value for the anisotropy of the dielectric constants .DELTA..epsilon.. The molecules in this liquid-crystal layer have a homeotropic or tilted homeotropic alignment in the switched-off state, i.e. are aligned substantially perpendicular to the electrode surfaces. Owing to the negative .DELTA..epsilon., realignment of liquid-crystal molecules parallel to the electrode surface takes place in the switched-on state.
In contrast to conventional ECB displays, in which the liquid-crystal molecules in the switched-on state have a parallel alignment with a uniform preferential direction over the entire liquid-crystal cell, this uniform parallel alignment in VAN and VAC displays is restricted only to small domains within the cell. Disclinations exist between these domains, which are also known as tilt domains.
As a consequence, VAN and VAC displays have a greater viewing-angle independence of the contrast and of the grey shades than conventional ECB displays. In addition, such displays are simpler to produce, since additional treatment of the electrode surface, such as, for example, by rubbing, for uniform alignment of the molecules is no longer necessary.
In contrast to VAN displays, the liquid-crystal media in VAC displays additionally comprise one or more chiral compounds, such as, for example, chiral dopants, which, in the switched-on state, produce a helical twist of the liquid-crystal molecules in the liquid-crystal layer by an angle of between 0 and 360.degree.. The twist angle in the preferred case is about 90.degree..
In particular for these novel VAN and VAC displays, special customized liquid-crystal media are required. For example, it has been found that the liquid-crystalline media of negative dielectric anisotropy disclosed hitherto, as described, for example, in EP 0 474 062, have low values for the voltage holding ratio (HR) after UV exposure. They are therefore not very suitable for use in the displays described above.